getting total for particular field with where syntax

i have a form where i get the details of particular Combobox. Pic 2.jpg attached. here on right side i got few textboxes say txt1..txt2 .... txt7.

on left side of this jpg i got the details of a particular project number.

when you see 3.jpg its an access table with details of different projects. under taetigkeit field there are different fields with corresponding values for Stunden field.

now when i select a particular Project to show me the details this 2.jpg form appears with all the details corresponding to this project.
now i need the total stunden for each taetigkeit to be calculated and fill these txt1 txt2....so on till txt7. these txt1 till txt7 are these taetigkeit.